The Vernal Equinox - Spring Is Here!

Spring arrives in Mississauga today - astronomically, that is - at 12:57 pm.  When will the spring weather arrive? Hopefully, very soon.

What is the Vernal Equinox?  It is the day that the overhead Sun crosses the equator heading north. Last December 21st, the Sun was overhead for those standing on latitude minus 23.4 degrees. For us here in Mississauga, the Sun only spent 9 hours above the horizon - the shortest day of the year - and was only 23 degrees high in the south at its highest point.

 The Earth's axis is tilted 23.4 degrees. Today, March 20, we receive equal day and night, thus, the 'equinox'.

The Earth's axis is tilted 23.4 degrees. Today, March 20, we receive equal day and night, thus, the 'equinox'.

The two lines of latitude where the Sun reaches its most southerly position and northerly position - at -23.4 degrees and +23.4 degrees - are called the Tropic of Capricorn and the Tropic of Cancer. 

 A picture of a beer can sitting on the Tropic of Cancer on June 21, 1991 - the Sun is overhead - no shadow!

A picture of a beer can sitting on the Tropic of Cancer on June 21, 1991 - the Sun is overhead - no shadow!

The reason for this motion of the Sun is due to the tilt of the Earth's axis, 23.4 degrees to the vertical. In summer, the Sun is higher in the sky, the heat is more concentrated on the ground, the days are longer and we have summer.  In the winter, the heat is more spread out on the ground, the days are shorter and we have winter.

In the southern hemisphere, they see the opposite - they have summer during our winter (Christmas BBQ on the beach) and winter time during June-August.

For more on the Vernal Equinox, check out this EarthSky website.